U.S. patent number 5,017,531 [Application Number 07/469,727] was granted by the patent office on 1991-05-21 for silicon nitride ceramic sintered bodies.
This patent grant is currently assigned to NGK Insulators, Ltd.. Invention is credited to Issei Hayakawa, Tadaaki Matsuhisa, Mithuru Miyamoto, Noriyuki Ukai.
United States Patent |
5,017,531 |
Ukai , et al. |
May 21, 1991 |
Silicon nitride ceramic sintered bodies
Abstract
High density fine ceramic sintered bodies are disclosed, which
have a maximum pore diameter of not more than 10 .mu.m and a
porosity of not more than 0.5%. A process for producing such high
density fine ceramic sintered bodies comprises the steps of mixing
a ceramic raw material powder with a sintering aid, grinding the
resultant, mixture granulating and shaping the mixture, and firing
the shaped body. The granulated powder is once forcedly dried, and
upon necessity is added with water and/or sieved to obtain a
uniform granulated powder having a given amount of water.
Inventors: |
Ukai; Noriyuki (Nagoya,
JP), Hayakawa; Issei (Nagoya, JP),
Miyamoto; Mithuru (Kariya, JP), Matsuhisa;
Tadaaki (Kasugai, JP) |
Assignee: |
NGK Insulators, Ltd.
(JP)
|
Family
ID: |
27453331 |
Appl.
No.: |
07/469,727 |
Filed: |
January 24, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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295770 |
Jan 11, 1989 |
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129135 |
Dec 7, 1987 |
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Foreign Application Priority Data
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Dec 16, 1986 [JP] |
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61-297756 |
Jan 8, 1987 [JP] |
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62-1132 |
Feb 18, 1987 [JP] |
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62-35233 |
Mar 31, 1987 [JP] |
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62-78228 |
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Current U.S.
Class: |
501/97.3;
501/104 |
Current CPC
Class: |
C04B
35/584 (20130101); C04B 35/593 (20130101); C04B
35/626 (20130101) |
Current International
Class: |
C04B
35/584 (20060101); C04B 35/626 (20060101); C04B
35/593 (20060101); C04B 035/58 () |
Field of
Search: |
;501/97,98,104 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0202899 |
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Nov 1986 |
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EP |
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9026974 |
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Feb 1984 |
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JP |
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61-97158 |
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May 1986 |
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JP |
|
Primary Examiner: Group; Karl
Attorney, Agent or Firm: Parkhurst, Wendel & Rossi
Parent Case Text
This is a continuation of application Ser. No. 07/295,770 filed
Jan. 11, 1989, now abandoned, which in turn is a Division of
application Ser. No. 07/129,135, filed Dec. 7, 1987.
Claims
What is claimed is:
1. A Si.sub.3 N.sub.4 sintered ceramic body, consisting essentially
of:
MgO and/or an Mg compound in an amount of 0.5 to 15.9% by weight
when calculated as MgO;
ZRO.sub.2 and/or a Zr compound in an amount of 0.5 to 13.9% by
weight when calculated as ZrO.sub.2 ;
Y.sub.2 O.sub.3 and/or a Y compound in an amount of 2 to 14.8% by
weight when calculated as Y.sub.2 O.sub.3 ; and
the remainder being Si.sub.3 N.sub.4 ;
wherein said sintered body has a maximum pore diameter of not more
than 10 .mu.m, a porosity of not more than 0.5%, and a four point
flexural strength at room temperature of not less than 100
kg/mm.sup.2.
2. The ceramic sintered body according to claim 1, wherein said
sintered ceramic body has a Knoop hardness of not less than 15.5
Gpa.
3. The Si.sub.3 N.sub.4 sintered ceramic body of claim 1, said
sintered body being produced by the method comprising:
mixing Si.sub.3 N.sub.4 ceramic raw material powder with the Mg, Zr
and Y components;
grinding and granulating the resultant mixture;
forcedly drying the granulated powder;
adding water and/or sieving the dried powder, thereby forming a
uniform granulated powder having a given amount of water;
shaping said uniform powder to form a body;
firing said shaped body.
4. The Si.sub.3 N.sub.4 sintered ceramic body of claim 1, wherein
said sintered body has a rolling fatigue life of not less than
1.5.times.10.sup.7 cycles at a hertz stress of 500 kg/mm.sup.2 in a
six ball type thrust bearing tester.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to high density fine ceramic sintered
bodies useful for engine parts, gas turbine parts, mechanical
parts, wear resistant slide members, and the like, and a process
for manufacturing the same.
(2) Related Art Statement
In order to produce ceramic products, it is a conventional
practice, as shown in a flow chart of FIG. 5 by way of example, to
first mix a ceramic raw material with a sintering aid, and then
grind the mixture and pass through a sieve of 44 .mu.m to remove
foreign matter such as broken pieces of grinding media used for the
grinding. Then, after granulating, water is added to the granulated
powder as necessary, and the granulated powder is shaped by a mold
press or a cold isostatic press. The shaped body is finally
sintered at a given temperature to obtain a sintered product.
However, since positive measures are not taken to uniformly
disperse water in the granulated powder in the above-mentioned
conventional ceramic product-producing process, the amount of water
in the granulated powder is locally varied. As a result, pores are
formed in the shaped bodies due to nonuniform particle fracture
which is caused by nonuniform water distribution in the granulated
powder, so that such pores remain in the sintered products.
Consequently, ceramic sintered bodies having excellent mechanical
characteristics cannot be obtained.
Particularly, when ceramic sintered bodies are used as bearing
members, wear resistant members or slide members, pores and
hardness largely influences the use life thereof. Thus, in order to
obtain ceramic products having longer use life than before, it was
necessary to produce high hardness ceramic sintered products having
a smaller pore diameter and porosity. Among them, when they are
used as bearing materials, it is known that it is important to
improve the fatigue life of the materials. Thus, there has been a
demand to develop dense, high strength, and/or high hardness
ceramic materials to improve rolling fatigue life.
SUMMARY OF THE INVENTION
It is an object of the present invention to obviate the
above-mentioned drawbacks, and to provide dense ceramic sintered
bodies having high strength and/or high hardness, as well as a
process for producing the same.
The ceramic sintered bodies according to the present invention are
characterized in that the maximum pore diameter is not more than 10
.mu.m, and the porosity is not more than 0.5%. Preferably, four
point flexural strength at room temperature is not less than 100
kg/mm.sup.2 and/or Knoop hardness is not less than 15.5 GPa.
The process for producing the ceramic sintered bodies according to
the present invention comprises the steps of mixing a ceramic raw
material powder with a given sintering aid, grinding, granulating,
and shaping the resulting mixture. This process is characterized in
that after the granulation, uniform granulated powder having a
given amount of water is obtained by forcedly drying the granulated
powder and then as necessary adding water to the powder and/or
sieving, the powder through a sieve.
In order to obtain silicon nitride sintered bodies, it is
preferable that a silicon nitride raw material containing not less
than 90% by weight of .alpha.-silicon nitride is used and that
firing is affected pressurelessly (i.e. under atmospheric
pressure). More preferably, an obtained shaped body is
preliminarily treated, and then treated by hot isostatic pressing
(HIPing) in a nitrogen atmosphere.
In the above construction, a uniform granulated powder having no
variation in water content over the granulated particles can be
obtained by once forcedly drying the granulated powder, and as
necessary, adding water and/or passing the granulated powder
through a sieve.
That is, pores present between the particles can be reduced by
attaining a uniformly press-crushed state during shaping through
forcedly drying the granulated powder and adding water thereto at
need. As a result, when the thus obtained granulated powder is
shaped and fired, for instance, as to silicon nitride sintered
bodies, high strength and high density ceramic sintered bodies
having a maximum pore diameter of not more than 10 .mu.m, porosity
of not more than 0.5%, four point flexural strength of not less
than 100 kg/mm.sup.2 at room temperature, and Knoop hardness of not
less than 15.5 GPa can be obtained even by pressureless
sintering.
These and other objects, features and advantages of the present
invention will be appreciated upon reading the following
description of the invention when taken in conjunction with the
attached drawings, with the understanding that some modifications,
variations, and changes of a same could be made by the skilled
person in the art to which the invention pertains without departing
from the spirit of the invention or the scope of claims appended
hereto.
BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS
For a better understanding of the invention, reference is made to
the attached drawings, wherein:
FIG. 1 is a flow chart illustrating an example of the production
process according to the present invention;
FIG. 2 is a graph showing the relationship between four point
flexural strength of the sintered bodies according to the present
invention and the granulated powder-drying temperature;
FIG. 3 is a graph showing the relationship between four point
flexural strength of the sintered bodies according to the present
invention and the amount of water added to the granulated
powder;
FIG. 4 is a graph showing the relationship between four point
flexural strength of the sintered bodies according to the present
invention and the sieve opening after grinding; and
FIG. 5 is an example of a flow chart of a conventional process for
the production of a silicon nitride sintered bodies.
DETAILED DESCRIPTION OF THE INVENTION
In order to obtain silicon nitride sintered bodies according to the
present invention, any sintering aid may be used so long as it can
make the silicon nitride dense, strong and hard. However, it is
preferable to use MgO, ZrO.sub.2, Y.sub.2 O.sub.3 and/or a compound
of Mg, Zr or Y which is converted to MgO, ZrO.sub.2, or Y.sub.2
O.sub.3 by heating, respectively. The reason is that MgO,
ZrO.sub.2, Y.sub.2 O.sub.3 or the Mg-, Zr- or Y-compound promotes a
phase transformation to elongated or rod-like .beta.-silicon
nitride crystals which are advantageous for attaining high strength
and the Zr compound strengthens a intergranular phase when present
in the intergranular phase during the sintering. The reason why it
is preferable that the Mg compound is added in an amount of from
0.5 to 15% by weight when calculated as MgO, and that the Zr
compound is added in an amount of from 0.5 to 13% by weight when
calculated as ZrO.sub.2, and that Y compound is added in an amount
of from 2 to 15% by weight when calculated as Y.sub.2 O.sub.3, is
that if they fall outside these respective ranges, above-mentioned
effects are reduced. Further, in the case of silicon nitride
pressurelessly sintered bodies, it is preferable that not less than
90% by weight of silicon nitride is .beta.-silicon nitride
crystals. This is because if it is less than 90% by weight, it is
difficult to attain high strength.
The forcedly drying temperature is preferably in a range of from
60.degree.to 100.degree. C. The reason is that if it is less than
60.degree. C., it is difficult to attain a desired dried state,
while if it is over 100.degree. C., it is also difficult to attain
a uniform press-crushed state of the granulated powder due to
hardening of an auxiliary used in spray drying.
Furthermore, it is preferable that the ground raw material is
passed through a sieve having a sieve opening of not more than 32
.mu.m before granulating or the forcedly dried and water-added
granulated powder is passed through a sieve having a sieve opening
of not more than 250 .mu.m. The reason is that coarse particles
after grinding and foreign matters contained in the raw materials
cannot effectively be removed by using a sieve having a larger
sieve opening than the above, so that it is difficult to maintain
uniformity of the granulated powder.
In addition, the amount of water added to the granulated powder is
preferably in a range from 0.5 to 5% by weight. The reason is that
if it is less than 0.5% by weight, water may not be uniformly
distributed among the granulated particles to cause variation in
the water content, while if it is over 5% by weight, water may ooze
out from the surface of the shaped body during shaping to cause
nonuniform water distribution in the shaped body.
The granulation is preferably affected by spray drying. The reason
is that granulated powder of which packing density can be increased
during the shaping can be obtained thereby.
Polyvinyl alcohol (PVA), polyethylene glycol (PEG), methyl
cellulose (MC), and stearic acid are preferred as an auxiliary used
in the spray drying. The reason is that the granulated powder is
difficult to harden or break by forcedly drying it and/or adding
water thereto when such as auxiliary is used.
It takes a long time to mix and grind a raw powder having an
average particle size of more than 2 .mu.m. During such a long time
of mixing and grinding, there is a possibility that impurities may
enter the raw material due to wearing during the grinding so that
characteristics of the sintered body are deteriorated and an effect
of densifying the sintered body is lost. Thus, it is preferable to
use fine raw materials having the average particle size of not more
than 2 .mu.m and more preferably not more than 1 .mu.m.
When the thus obtained shaped body is preliminarily treated and is
further treated by hot isostatic pressing (HIPing) in an inert gas
atmosphere, higher density, higher strength, and/or higher hardness
can be attained. Such treatments are preferable because, for
instance, a maximum pore diameter of not more than 10 .mu.m and a
porosity of not more than 0.3% can be attained. As an inert gas,
nitrogen, argon, or the like is used.
Furthermore, it is preferable to use a silicon nitride raw material
containing not less than 90%, more preferably, not less than 95% of
.alpha.-silicon nitride in the production of the silicon nitride
sintered bodies, because a sufficient amount of needle-like
.beta.-silicon nitride crystals which lead to high strength during
sintering are precipitated owing to the
.alpha..fwdarw..beta.transformation.
In a HIP sintered body as a preferred embodiment according to the
present invention, steps of forcedly drying, shaping, preliminarily
treating and hot isostatic press treatment are affected in this
order. In this process, the preliminary treatment is broken down
into two kinds: a primary sintering method in which the shaped body
is primarily fired and an encapsulating method in which a shaped
body is sealed in a capsule. In the primary sintering method of the
preliminary treatment, the shaped body is primarily fired,
preferably at 1,400.degree.to 1,600.degree. C. in an inert gas
atmosphere at ordinary pressure. If the firing temperature is less
than 1,400.degree. C., open pores do not disappear even after
firing, so that dense sintered bodies cannot be obtained even after
the hot isostatic press treatment. On the other hand, if it is more
than 1,600.degree. C., a decomposition reaction proceeds during
sintering, so that dense, high strength, and/or high hardness
sintered bodies cannot be obtained even after the HIP treatment,
either.
Meanwhile, there are two methods for the encapsulating step. That
is, a shaped body is sealed in a vacuum-evacuated glass vessel
preferably consisting mainly of SiO.sub.2 before the HIP treatment.
Alternatively, a shaped body is buried in a glass powder before the
HIP, and the glass powder is melted as the temperature rises in the
HIP treatment to encapsulate the shaped body. The reason why glass
is preferred as an encapsulating material is that glass has better
deforming ability and sealability as a capsule during the hot
isostatic press treatment.
FIG. 1 shows a flow chart of an example of the production process
according to the present invention. First, a ceramic raw material
having an average particle size of not more than 2 .mu.m is mixed
and ground with a sintering aid. The thus obtained mixture is
passed through a sieve having a sieve opening of, preferably, not
more than 32 .mu.m to remove foreign matters and coarse particles
such as broken pieces of grinding media used for grinding. Any
sintering aid may be used as long as it can densify and strengthen
intended ceramic materials. Then, the mixture is granulated to
obtain a granulated powder containing about 1% by weight. Water the
granulated powder is sieved similarly in a conventional manner. The
resulting granulated powder is forcedly dried, preferably, at a
temperature range from 60.degree.to 100.degree. C. to obtain a
uniform granulated powder containing water at a low variation in an
amount of from 0.2 to 0.5% by weight. Next, 0.5 to 5.0% by weight
of water is added to the granulated powder as necessary to obtain a
granulated powder having a uniform water content, and a final
granulated powder is obtained by removing coarse particles
coagulated by the addition of water, with a sieve having a sieve
opening of not more than 250 .mu.m. High strength, high density
and/or high hardness ceramic sintered bodies having the
characteristics of the present invention are obtained by shaping
the thus obtained granulated powder in an ordinary manner and
firing the resulting shaped body at ordinary pressure.
In the following, examples of the present invention will be
explained. These examples are merely given in illustration of the
invention, but should never be interpreted to limit the scope
thereof.
EXAMPLE 1
Into 100 parts by weight of a Si.sub.3 N.sub.4 powder having an
average particle size of 0.5 .mu.m were added and mixed 3 parts by
weight of MgO, 1 part by weight of ZrP.sub.2, 4 parts by weight of
CeO.sub.2, and 1 part by weight of SrO as sintering aids. After 60%
by weight of water was added to the thus obtained mixture together
with grinding media having a diameter of 5 to 10 mm, the mixture
was mixed and ground for 4 hours by a batch grinder.
Next, the mixed and ground slurry was passed through a JIS standard
sieve with a sieve opening of 32 .mu.m, which was added and mixed
with 2% by weight of PVA and 0.2% by weight of stearic acid used as
auxiliaries in spray drying. Thereafter, a granulated powder having
an average particle size of 80 .mu.m and a water content of from
0.5 to 1.0% by weight was obtained by spray-drying.
Further, the granulated powder was forcedly dried at a drying
temperature shown in Table 1 for 24 hours by using a thermostatic
drier, and water was added thereto as necessary. When water was
added, the powder was passed through a JIS standard sieve having a
sieve opening shown in Table 1, thus obtaining granulated powders
(Sample Nos. 1 to 8). The granulated powder was shaped at a
pressure of 3 ton/cm.sup.2 by cold isostatic press, thereby
obtaining a shaped body of 60mm.times.60mm.times.6mm.
Then, after the shaped body was dewaxed at a temperature of
500.degree. C. for 3 hours, it was pressurelessly sintered at a
temperature of 1,700.degree. C. for 1 hour in a nitrogen gas
atmosphere, thus obtaining high strength silicon nitride sintered
bodies according to the present invention (Samples Nos. 1 to 8).
Apart from the above, granulated powders in Sample Nos. 9-11 were
prepared as a Comparative Example of the present invention under
production conditions shown in Table 1 with no forcedly drying, and
were shaped and fired under similar conditions, thereby obtaining
sintered bodies.
Then, with respect to the thus obtained sintered bodies, flexural
strength, maximum pore diameter, and porosity were measured.
Measurement results are shown in Table 1. The flexural strength was
measured according to a four point flexural strength test method of
fine ceramic flexural strength-testing in JIS R-1601. The maximum
pore diameter and the porosity were measured with respect to a
mirror-polished surface of the sintered body by means of an optical
microscope at 400.times.magnification. The maximum width of pore
was taken as the diameter of the pore, while the maximum diameter
of 1,000 pores measured was taken as the maximum pore diameter.
With respect to the porosity, the total pore area was determined by
actually measuring areas of the above 1,000 pores in the above
measurement, and the porosity was determined as a value obtained by
dividing the total pore area by a total area as viewed in the
measurement.
TABLE 1
__________________________________________________________________________
Producing conditions Measurement results Forcedly Sieve opening
Maximum drying Amount of after water Flexural pore temperature
water added added strength diameter Porosity Sample No.
(.degree.C.) (wt %) (.mu.m) (kg/mm.sup.2) (.mu.m) (%)
__________________________________________________________________________
Present 1 45 -- -- 91 10 0.5 inven- 2 45 3 325 95 7 0.3 tion 3 60
0.5 250 99 7 0.2 4 80 3 149 113 6 0.2 5 80 5 250 104 7 0.2 6 100 --
-- 95 8 0.4 7 120 3 149 97 7 0.4 8 140 6 325 92 9 0.3 Compar- 9 --
-- -- 81 23 2.7 ative 10 -- 5 325 85 18 0.8 example 11 -- 3 -- 79
32 3.6
__________________________________________________________________________
As compared with the Comparative Examples, it is clear from Table 1
that the sintered bodies, according to the present invention, using
a mixed raw material having been forcedly dried, added with water
and sieved at need, are less porous, superior sintered bodies
having much higher strength.
EXAMPLE 2
Into a pot made of zirconia were placed 100 parts by weight of
ZrO.sub.2 powder having an average particle size of 1.8 .mu.m, 5
parts by weight of a stabilizer Y.sub.2 O.sub.3, 2 parts by weight
of a sintering aid Al.sub.2 O.sub.3, and 100 parts by weight of
water, which were mixed and ground. As shown in Table 2, the
grinding was affected for 1 hour, 10 hours, or 30 hours. Then, 1%
by weight of an auxiliary PEG to be used in spray drying was added
to the thus mixed and ground slurry, which was spray dried to
prepare a granulated powder. The thus obtained granulated powder
was sampled, and was forcedly dried at a temperature shown in Table
2 for 30 hours by using a dry air drier. After water was added at
need, the granulated powder was shaped at 1.5 ton/cm.sup.2 by a
cold isostatic press machine to obtain a shaped body of
60.times.60.times.6mm. Thereafter, the shaped body was dried,
dewaxes, and fired at 1,400.degree. C. in air, thus obtaining
sintered bodies according to the present invention (Sample Nos. 12
to 18). Sintered bodies (Sample Nos. 19 to 21) were obtained as
Comparative Examples utilized the same process except that no
forcedly drying was affected. Further, a sintered body (Sample No.
22) was obtained as a Comparative Example by the same process
except that ZrO.sub.2 having the average particle size of 3 .mu.m
was used and the grinding was carried out for 48 hours without
being accompanied by forcedly drying.
Then, the flexural strength of the thus obtained sintered bodies
and the relative density of the shaped bodies were measured, and
are shown in Table 2. The flexural strength was measured by the
same method as in Example 1, and the relative densities of the
shaped body wa determined as follows: ##EQU1##
As a result, it is seen from Table 2 that the invention products
have higher relative density than the shaped bodies and greatly
improved strength. Thus, the high strength zirconia sintered bodies
were obtained by the production process according to the present
invention. Comparative Example 22 having an average particle size
greater than 2 .mu.m was deteriorated in a flexural strength.
TABLE 2
__________________________________________________________________________
Relative Average Amount density Grind- particle size of of Maximum
ing after mixing Forcedly water Flexural shaped pore time and
grinding drying added strength body diameter Porosity Sample No.
(hr) (.mu.m) (.degree.C.) (%) (kg/mm.sup.2) (%) (.mu.m) (%)
__________________________________________________________________________
Present 12 1 1.5 60 3 85 55 9 0.4 inven- 13 1 1.5 100 -- 81 54 9
0.3 tion 14 1 1.5 120 -- 79 53 10 0.4 15 10 0.7 60 -- 90 55 8 0.4
16 10 0.7 80 3 97 56 5 0.2 17 30 0.4 60 2 95 55 7 0.3 18 30 0.4 100
4 93 55 7 0.3 Compar- 19 1 1.5 -- -- 73 51 15 0.8 ative 20 10 0.7
-- -- 80 51 11 0.6 example 21 30 0.4 -- -- 76 48 12 0.7 22 48 1.4
-- -- 75 52 12 0.7
__________________________________________________________________________
EXAMPLE 3
To .alpha.-silicon nitride powder having an average particle size
of 0.5 .mu.m were mixed powdery MgO, ZrO.sub.2, and Y.sub.2 O.sub.3
as sintering aids at rates of 4% by weight, 3% by weight, and 6% by
weight, respectively. After 60% by weight of water was added to the
thus obtained mixture, together with grinding media of 5 to 10 mm
in diameter, the mixture was mixed and ground for 4 hours in a
batch grinder.
Then, the mixed and ground slurry was passed through a JIS standard
sieve with a sieve opening of 32 .mu.m; and 2% by weight of PVA and
0.2% by weight of stearic acid were added to the slurry as a spray
drying auxiliaries, which was spray dried to obtain a granulated
powder having the average particle size of 80 .mu.m and a water
content of from 0.5 to 1.0% by weight.
Further, the granulated powder was forcedly dried at a temperature
shown in Table 3 for 24 hours by using a thermostatic drier, and
water was added thereto at need. When water was added, the
granulated powder was sieved with a JIS standard sieve of sieve
opening shown in Table 3, thus obtaining granulated powders (Sample
Nos. 31 to 38). The granulated powder was shaped at a pressure of
2.5 ton/cm.sup.2 by cold isostatic press, thus obtaining a shaped
body of 60mm.times.60mm.times.6 mm.
Then, after the shaped body was dewaxed at a temperature of
500.degree. C. for 3 hours, the shaped body was pressurelessly
sintered at a temperature of 1,700.degree. C. in a nitrogen gas
atmosphere for 1 hour, thus obtaining high strength silicon nitride
sintered bodies according to the present invention (Sample Nos. 31
to 38). Apart from the above, granulated powders of Sample Nos. 39
to 41 were prepared as Comparative Examples of the present
invention under the production conditions shown in Table 3 with no
forcedly drying, and the granulated powders were shaped and fired
under the same conditions, thereby obtaining sintered bodies.
Then, flexural strength, maximum pore diameter, and porosity of the
sintered bodies and a ratio of .beta.-silicon nitride crystals in
the sintered body were measured in the same manner as in Example 1;
Measurement results are shown in Table 3.
TABLE 3
__________________________________________________________________________
Producing conditions Measurement results Forcedly Sieve opening
Maximum Ratio of drying Amount of after water Flexural pore
crystals temperature water added added strength diameter Porosity
of .beta.-Si.sub.3 N.sub.4 Sample No. (.degree.C.) (wt %) (.mu.m)
(kg/mm.sup.2) (.mu.m) (%) (%)
__________________________________________________________________________
Present 31 45 -- -- 101 9 0.5 100 inven- 32 45 3 325 106 6 0.3 100
tion 33 60 0.5 250 110 5.5 0.2 100 34 80 3 149 125 5 0.2 100 35 80
5 250 115 6 0.2 100 36 100 -- -- 105 8 0.4 100 37 120 3 149 108 6
0.4 100 38 140 6 325 102 8 0.3 100 Compar- 39 -- -- -- 90 22 2.7
100 ative 40 -- 5 325 94 17 0.8 100 example 41 -- 3 -- 88 31 3.6
100
__________________________________________________________________________
As is clear from Table 3, the sintered bodies according to the
present invention using the mixed powder being forcedly dried, and
added with water and sieved at need, were sintered bodies which had
extremely higher strength and were less porous as compared with the
Comparative Examples.
EXAMPLE 4
In order to examine the influences of the composition of sintered
bodies and the sieve opening after grinding, granulated powders
(Sample Nos. 42 to 55) were each obtained by forcedly drying
granulated powder at a temperature of 80.degree. C. for 24 hours in
the same manner as in Example 3, adding 4% by weight of water
thereto, and passing it through a sieve having a sieve opening of
149 .mu.m. The thus obtained granulated powder was shaped and
dewaxed in the same manner as in Example 3, which was
pressurelessly sintered in a nitrogen gas atmosphere at an optimum
selected firing temperature (1,600.degree.to 1,800.degree. C.) as
giving a ratio of .beta.-silicon nitride crystals being not less
than 90% by weight, thereby obtaining high strength silicon nitride
sintered bodies according to the present invention (Sample Nos. 42
to 55). Results are shown in Table 4.
TABLE 4
__________________________________________________________________________
Producing conditions Sieve Measurement results opening Maxi- Ratio
of after Firing mum crystals grind- temper- Composition of Flexural
pore Por- of Mixing rate (wt %) ing ature sintered body (wt %)
strength diameter osity .beta.-Si.sub.3 N.sub.4 Sample No. Si.sub.3
N.sub.4 Y.sub.2 O.sub.3 MgO ZrO.sub.2 (.mu.m) (.degree.C.) Si.sub.3
N.sub.4 Y.sub.2 O.sub.3 MgO ZrO.sub.2 (kg/mm.sup.2) (.mu.m) (%) (%)
__________________________________________________________________________
Present 42 95.0 4.0 0.5 0.5 32 1800 94.8 4.1 0.5 0.5 116 8.5 0.3 99
inven- 43 90.0 2.0 1.0 7.0 32 1750 89.7 2.0 1.0 6.8 119 7 0.4 100
tion 44 89.0 6.0 4.0 1.0 25 1750 89.0 5.9 4.1 0.9 128 8 0.2 100 45
87.0 6.0 4.0 3.0 20 1700 86.8 5.9 4.0 3.1 130 6 0.2 100 46 84.0 7.0
7.0 2.0 20 1700 83.9 6.9 7.0 2.1 125 5.5 0.3 100 47 77.0 6.0 4.0
13.0 25 1700 76.8 5.9 3.8 12.9 124 6 0.3 97 48 74.0 15.0 4.0 7.0 25
1600 73.7 14.7 4.0 6.8 120 5 0.4 95 49 70.0 2.0 15.0 13.0 32 1700
70.0 2.0 14.8 13.0 121 6 0.3 93 50 75.0 6.0 16.0 3.0 32 1650 75.0
6.0 15.9 3.0 109 8 0.3 93 51 76.0 6.0 4.0 14.0 44 1700 75.8 6.1 3.8
13.9 105 9.5 0.4 92 52 78.0 16.0 4.0 3.0 44 1700 78.0 14.8 3.9 3.0
105 9 0.4 96 53 85.0 1.0 7.0 7.0 20 1750 84.9 1.0 7.0 6.9 100 5.5
0.2 88 54 89.0 7.0 4.0 0 25 1700 88.7 7.0 3.9 0 102 6 0.3 89 55
90.0 7.0 0 3.0 25 1700 89.9 7.1 0 3.0 107 6 0.2 85
__________________________________________________________________________
As is clear from Table 4, among the sintered bodies of the
invention, those in which: the granulated powder has been passed
through a sieve of 32 .mu.m or less; the mixed raw material
contained from 0.5 to 15% by weight of MgO; the mixed raw material
contained from 0.5 to 13% by weight of ZrO.sub.2 ; the mixed raw
material contained from 2 to 15% by weight Y.sub.2 O.sub.3 ; or not
less than 90% by weight of .beta.-silicon nitride crystals were
contained in the sintered body, are preferred.
In order to facilitate understanding of the results in the above
Examples 3 and 4, FIGS. 2 and 3 show the relationship between the
four point flexural strength of sintered bodies obtained according
to the present invention and the forcedly drying temperature of
granulated powders, and the relationship between the four point
flexural strength of the sintered bodies and the amount of water
added to a granulated powder, respectively, while FIG. 4 showing
the relationship between the four point flexural strength of the
sintered bodies and the sieve opening of the sieve used after the
grinding.
EXAMPLE 5
To .alpha.-silicon nitride powder having the average particle size
of 0.5 .mu.m were mixed powdery MgO, ZrO.sub.2, and Y.sub.2 O.sub.3
as sintering aids at rates of 4% by weight, 2% by weight, and 7% by
weight, respectively. After 60% by weight of water was added to the
mixture, the mixture was mixed and ground by a batch grinder. The
thus ground mixture was passed through a sieve with a sieve opening
of 20 .mu.m, thereby obtaining a slurry of the ground particles
having an average particle size of 0.7 .mu.m. Then, 2% by weight of
polyvinyl alcohol (PVA) was added to the slurry, which was
converted to a granulated powder by using a spray drier.
Further, the granulated powder was forcedly dried at a temperature
shown in Table 5 for 24 hours by using a thermostatic drier, and
water was added thereto upon necessity. When water was added, the
powder was sieved by using a JIS standard sieve having a sieve
opening of Table 5, thus obtaining granulated powders (Sample Nos.
61 to 69). The granulated powder was shaped at a pressure of 5
ton/cm.sup.2 by cold isostatic pressing, thereby preparing a shaped
body of 65 mm(.phi.).times.50 mm(length).
Thereafter, the shaped body was dewaxed at a temperature of
500.degree. C. for 3 hours, and pressurelessly sintered at a
temperature of 1,460.degree. C. for 6 hours in a nitrogen (N.sub.2)
atmosphere (primary sintering step). Then, the primarily sintered
body was treated at a temperature of 1,700.degree. C. under a
pressure of 400 atms in an N.sub.2 atmosphere by hot isostatic
pressing (HIPing), thus obtaining sintered bodies according to the
present invention (Sample Nos. 61 to 69). Apart from the above,
sintered bodies (Sample Nos. 70 to 72) were prepared as Comparative
Examples of the present invention under production conditions shown
in Table 5 with no forcedly drying.
Properties of the thus obtained sintered bodies are shown in Table
5.
Knoop hardness was measured according to JIS Z2251 after a test
sample was held under a load of 300 g for 15 seconds.
Rolling fatigue life was evaluated by cutting off a round disc of
50 mm(.phi.).times.10 mm(thick) from the sintered sample, mirror
polishing the surface of the round disc, and subjecting it to a
rolling fatigue test at a hertz stress of 500 kg/mm.sup.2 in a six
ball type thrust bearing tester.
As is clear from Table 5, the HIP sintered bodies according to the
present invention, using the mixed raw material being forcedly
dried and added with water and further sieved at need, are sintered
bodies which are extremely less porous and have excellent
mechanical strengths, as compared with the Comparative
Examples.
TABLE 5
__________________________________________________________________________
Producing conditions Measurement results Forcedly Amount Sieve
opening Maximum Rolling drying of water after water pore Knoop
fatigue temperature added added diameter Porosity hardness life
Sample No. (.degree.C.) (wt %) (.mu.m) (.mu.m) (%) (GPa) (cycle)
__________________________________________________________________________
Present 61 40 -- -- 8.0 0.25 15.5 1.5 .times. 10.sup.7 inven- 62 60
-- -- 5.0 0.19 16.1 6 .times. 10.sup.7 tion 63 60 0.5 149 3.5 0.12
16.7 40 .times. 10.sup.7 64 70 3 325 1.0 0.01 17.2 >110 .times.
10.sup.7 65 80 5 250 2.5 0.02 17.0 >80 .times. 10.sup.7 66 90 --
-- 4.0 0.10 16.5 30 .times. 10.sup.7 67 100 3 325 4.0 0.09 16.5 35
.times. 10.sup.7 68 120 6 250 5.5 0.20 15.8 9 .times. 10.sup.7 69
140 5 149 7.0 0.23 15.7 3 .times. 10.sup.7 Compar- 70 -- 3 149 11.5
0.43 14.5 0.4 .times. 10.sup.7 ative 71 -- 4 -- 13.0 0.42 14.7 0.25
.times. 10.sup.7 example 72 -- -- -- 16.0 0.58 13.8 0.07 .times.
10.sup.7
__________________________________________________________________________
EXAMPLE 6
In order to examine the influences of sieving after grinding and
the average particle size after grinding, except that the kind and
the amount of addition of sintering aids were changed as shown in
Table 6 (in the same manner as in Example 5) the granulated powder
was forcedly dried at 80.degree. C. for 24 hours, 4% by weight of
water was added thereto, and the powder was passed through a sieve
with an opening of 149 .mu.m, thus obtaining granulated powders
(Sample Nos. 73 to 77).
After the thus obtained granulated powder was shaped and dewaxed in
the same manner as in Example 5, the powder was sealed in a silica
glass capsule under a vacuum. Then, the capsule was placed in a HIP
apparatus, which was HIP-treated at a temperature of 1,600.degree.
C. under a pressure of 1,500 atms, thus obtaining silicon nitride
sintered bodies (Sample Nos. 73 to 77). With respect to the thus
obtained sintered bodies, the rolling fatigue test was affected
similarly as in Example 5, and further, a wear resistance test was
affected.
In the wear resistance test, a cylindrical sample of 15
mm(.phi.).times.15 mm(length) was cut off from each of Sample Nos.
73 to 77, and abraded with a #140 diamond grind stone, which was
subjected to the wear resistance test by using a ball mill. Test
conditions were that an alumina vessel having an inner diameter of
120 mm(.phi.) was used, and rotated at 120 rpm.
A slurry liquid in which #100 silicon carbide powder and water were
mixed at a weight ratio of 1:1 was filled up to a half of the
vessel. Then, five of the above-prepared samples of 15
mm(.phi.).times.15 mm(length) were placed in the slurry, and then
subjected to the wear resistance test for 24 hours.
A wear amount was determined from changes in weight and dimension
before and after the test. Results in the rolling fatigue test and
the wear resistance test are shown in Table 6.
It is seen from Table 6 that among the invention products, those in
which the ground powder had been passed through the sieve of not
more than 32 .mu.m; or the average particle size after the grinding
was not more than 1 .mu.m are preferred.
TABLE 6
__________________________________________________________________________
Producing conditions Average Sieve particle Measurement results
opening size Maximum Rolling after water after pore Wear fatigue
Composition added grinding diameter Porosity amount life Sample No.
(wt %) (.mu.m) (.mu.m) (.mu.m) (%) (kg/cm.sup.2) (cycle)
__________________________________________________________________________
Present 73 92% Si.sub.3 N.sub.4 -2% SrO- 44 0.7 9.0 0.30 0.10 2
.times. 10.sup.7 inven- 3% MgO-3% CeO.sub.2 tion 74 92% Si.sub.3
N.sub.4 -2% SrO- 32 0.7 4.0 0.11 0.03 50 .times. 10.sup.7 3% MgO-3%
CeO.sub.2 75 92% Si.sub.3 N.sub.4 -2% SrO- 17 0.7 3.0 0.06 0.01 70
.times. 10.sup.7 3% MgO-3% CeO.sub.2 76 91% Si.sub.3 N.sub.4 -5%
y.sub.2 O.sub.3 - 25 0.8 5.0 0.15 0.06 30 .times. 10.sup.7 4%
Al.sub.2 O.sub.3 77 91% Si.sub.3 N.sub.4 -5% y.sub.2 O.sub.3 - 25
1.4 7.5 0.25 0.09 5 .times. 10.sup.7 4% Al.sub.2 O.sub.3
__________________________________________________________________________
EXAMPLE 7
To a silicon nitride raw powder having a average particle size of
0.7 .mu.m and containing 97% by weight of .alpha.-silicon nitride
were mixed powdery MgO, SrO, and CeO.sub.2 at rates of 3.5% by
weight, 1.5% by weight, and 5% by weight, respectively. After 60%
by weight of water was added to the mixture, the mixture was mixed
and ground by a batch grinder and then passed through a sieve
having a sieve opening of 20 .mu.m, thereby obtaining a slurry
containing powder having an average particle size of 0.5 .mu.m.
Then, 2% by weight of polyvinyl alcohol (PVA) was added to the
slurry, which was converted to a granulated powder by using a spray
drier.
Further, the granulated powder was forcedly dried at a temperature
shown in Table 7 for 24 hours by using a thermostatic drier, and
upon necessity, wear was added and the mixture was sieved through a
JIS standard sieve having a sieve opening shown in Table 7, thus
obtaining granulated powders (Sample Nos. 81 to 88). The granulated
powder was shaped at a pressure of 3 ton/cm.sup.2 by cold isostatic
pressing, thereby preparing a shaped body of 65 mm(.phi.).times.50
mm(length).
Thereafter, the shaped body was dewaxed at a temperature of
500.degree. C. for 3 hours, and then pressurelessly sintered at a
temperature of 1,460.degree. C. for 6 hours in a nitrogen (N.sub.2)
atmosphere (primary sintering step). Then, the primarily sintered
body was treated by hot isostatic pressing (HIPing) at a
temperature of 1,680.degree. C. under a pressure of 400 atms in an
N.sub.2 atmosphere, thus obtaining sintered bodies according to the
present invention (sample Nos. 81 to 88). Apart from the above,
sintered bodies (Sample Nos. 89 to 91) were prepared as Comparative
Examples of the present invention under production conditions shown
in Table 7 with no forcedly drying.
Properties of the thus obtained sintered bodies are shown in Table
7.
Flexural strength, maximum pore diameter, and porosity of the thus
obtained sintered bodies were measured in the same manner as in
Example 1, and measurement results are shown in Table 7.
As is clear from Table 7, the HIP sintered bodies according to the
present invention using the mixed raw material having been forcedly
dried, added with water and sieved at need, are sintered bodies
which are far less porous and have excellent mechanical strength,
as compared with the Comparative Examples.
TABLE 7
__________________________________________________________________________
Producing conditions Measurement results Forcedly Amount Sieve
opening Maximum 4-point drying of water after water pore flexural
temperature added added diameter Porosity strength Sample No.
(.degree.C.) (wt %) (.mu.m) (.mu.m) (%) (kg/mm.sup.2)
__________________________________________________________________________
Present 81 40 -- -- 9.0 0.35 121 invent- 82 60 0.5 149 4.5 0.17 125
tion 83 70 3 325 2.0 0.02 131 84 80 5 250 3.5 0.03 135 85 90 -- --
5.0 0.15 126 86 100 3 325 4.5 0.05 140 87 120 6 250 6.5 0.25 125 88
140 5 149 8.0 0.25 122 Compar- 89 -- 3 149 12.0 0.44 96 ative 90 --
4 -- 13.5 0.43 95 example 91 -- -- -- 16.5 0.60 92
__________________________________________________________________________
EXAMPLE 8
In order to examine the influences of sieving after grinding and
the average particle size after the grinding, an .alpha.-silicon
nitride raw material (containing 99.5% by weight of .alpha.-silicon
nitride) was mixed with sintering aids of which kinds and mixing
ratios are shown in Table 8. After 60% by weight of water was added
to the mixture, the mixture was ground and sieved at a sieve
opening as shown in Table 8, thereby obtaining a slurry. The
average particle size in the slurry is shown in Table 8. Then, 1%
by weight of polyvinyl alcohol and 0.5% by weight of stearic acid
were added to the thus obtained slurry, which was converted to a
granulated powder by using a spray drier. After the granulated
powder was dried at 70.degree. C. for 20 hours by using a drier and
4% by weight of water was added thereto, the powder was passed
through a sieve with an opening of 149 .mu.m. The resulting powder
was shaped at a pressure of 6 ton/cm.sup.2 by cold isostatic
pressing, thereby obtaining a shaped body of 30 mm(.phi.).times.60
mm(length). Then, the shaped body was dewaxed at 500.degree. C. for
3 hours, and then sealed in a silica glass capsule under a vacuum.
Next, the capsule was inserted into a HIP apparatus, and
HIP-treated at a temperature of 1,700.degree. C. under a pressure
of 1,500 atms, thus obtaining silicon nitride sintered bodies
(Sample Nos. 92 to 97).
As is clear from Table 8, among the invention products, those in
which the powder had been passed through a sieve of a sieve opening
of not more than 32 .mu.m after the grinding; or the average
particle size after grinding was not more than 1 .mu.m are
preferred.
TABLE 8
__________________________________________________________________________
Average particle size Sieve after opening mixing Maximum 4-point
after and pore flexural Sample Mixing rate (wt %) grinding grinding
diameter Porosity strength No. Si.sub.3 N.sub.4 Y.sub.2 O.sub.3 MgO
ZrO.sub.2 CeO.sub.2 (.mu.m) (.mu.m) (.mu.m) (%) (kg/mm.sup.2)
__________________________________________________________________________
92 91 5 3 1 0 44 0.6 8.0 0.20 127 93 91 5 3 1 0 25 0.6 3.5 0.08 145
94 91 5 3 1 0 17 0.6 2.5 0.06 157 95 88 5 5 0 2 32 0.4 5.0 0.11 135
96 88 5 5 0 2 32 0.9 7.5 0.25 123 97 88 5 5 0 2 32 1.4 8.0 0.30 120
__________________________________________________________________________
EXAMPLE 9
To an .alpha.-silicon nitride raw material containing 85% by
weight, 92% by weight, or 95% by weight of .alpha.-silicon nitride
were added 6% by weight of Y.sub.2 O.sub.3 and 4% by weight of
Al.sub.2 O.sub.3 as sintering aids, which were mixed, ground, and
sieved by a sieve with an opening of 32 .mu.m, thereby obtaining a
slurry. Then, 1% by weight of methyl cellulose was added to the
slurry, which was dried by a drier to prepare a granulated powder.
The granulated powder was further dried by a thermostatic drier at
100.degree. C. for 30 hours, and shaped at a pressure of 3
ton/cm.sup.2 by cold isostatic pressing, thereby obtaining a shaped
body of 30 mm (.phi.).times.50 mm(length). Then, the shaped body
was dewaxed at 450.degree. C. for 5 hours and sealed in a silica
glass capsule under a vacuum. Next, the capsule was inserted into a
HIP apparatus, and HIP-treated at a temperature of 1,600.degree.,
1,650.degree. , or 1,700.degree. C. under a pressure of 1,000 atms
for 30 minutes, thus obtaining silicon nitride sintered bodies
(Sample Nos. 98 to 104) as shown in Table 9.
TABLE 9 ______________________________________ Content of Content
of .alpha.-silicon Treating .beta.-silicon nitride temper- nitride
4-point in raw ature in sintered flexural material in HIP body
strength Sample No. (%) (.degree.C.) (%) (kg/mm.sup.2)
______________________________________ Example 98 95 1600 88 129 99
1650 93 135 100 1700 100 141 101 92 1650 93 123 102 1700 100 127
Compar- 103 85 1650 95 104 ative 104 1700 100 105 example
______________________________________
As is clear from Table 9, it was revealed that the sintered bodies
using a silicon nitride raw material containing not less than 90%
by weight of .alpha.-silicon nitride have higher strength.
As is clear from the foregoing, according to the present invention,
high strength, high density and/or high hardness fine ceramic
sintered bodies having excellent mechanical strength with smaller
maximum pore diameter and porosity can industrially be obtained at
an inexpensive cost due to a synergistic effect of forcedly drying
the granulated powder, and water incorporation and/or sieving at
need, irrespective of the HIP treatment and the pressureless
sintering. Consequently, the ceramic sintered bodies according to
the present invention can be applied to, for instance, high
temperature bearings, engine parts, gas turbine parts, and the
like, and thus have extremely great industrial value.
* * * * *